1. Field of the Invention
The present invention relates to a method of evaluating tuneable lasers and therewith discover systematically good operation points.
The method can be used to evaluate and select lasers with respect to wavelength coverage already at an early stage, and systematically discover good operation points.
2. Description of the Related Art
Tuneable semiconductor lasers have a number of different sections through which current is injected, typically three or four such sections. The wavelength, power and mode purity of the lasers can be controlled by adjusting the current injected into the various sections. Mode purity implies that the laser is tuned to an operation point, i.e. tuned to a combination of the three or four injected drive currents, which is characterized in that the laser is distanced from a combination of the drive currents where so-called mode jumps take place and where lasering is stable and side mode suppression is high.
Special wavelength controls are required with different applications. For instance, in the case of sensor applications it must be possible to tune the laser continuously, so as to avoid mode jumps as far as possible. In the case of telecommunications applications, it is necessary that the laser is able to retain its wavelength to a very high degree of accuracy and over very long periods of time, after having set the drive currents and the temperature. A typical accuracy in this respect is 0.1 nanometer and a typical time period is 20 years.
In order to be able to control the laser, it is necessary to map the behavior of the laser as a function of the various drive currents. This is necessary prior to using the laser after its manufacture.
Mapping of the behavior of a laser is normally effected by connecting the laser to different measuring instruments and then varying the drive currents systematically. Such instruments are normally power meters, optical spectrum analyzers for measuring wavelength and sidemode suppression, and line width measuring devices. This laser measuring process enables all of these parameters to be fully mapped as a function of all different drive currents.
One problem is that lasers exhibit hysteresis. As a result of the hysteresis, the laser will deliver different output signals in the form of power and wavelength in respect of a given drive current set-up, i.e., with respect to a given operation point, depending on the path through which the laser has passed with respect to the change in said drive currents, in order to arrive at the working point in question. Thus, this means that a given drive current set-up will not unequivocally give the expected wavelength or power.
The present invention relates to a method which results in ensuring that unequivocal operation points are obtained.
Accordingly, the present invention relates to a method of evaluating a tuneable laser and determining suitable laser operation points. The laser includes two or more tuneable sections in which injected current can be varied, said sections including at least one reflector section and one phase section. The method includes varying the current injected through the reflector section, i.e., the reflector current, at different constant currents injected through respective remaining tuneable sections. The laser power output at the front or the rear mirror of the laser is measured, and the reflector current is swept in one direction and then in the opposite direction back to its starting value while measuring and storing the power. The power difference with one and the same reflector current is calculated, but in the different sweep directions. Those combinations which give rise to a power difference that falls below a predetermined level are detected and stored as hysteresis-free current combinations.